In current digital still cameras and video cameras, the grayscale range (about 10 to 12 bits) of an image signal in input and processing systems is set larger than the grayscale range of the final output signal (normally 8 bits) to prevent degradation of the image quality caused by cancellation of digits during digital signal processing. In such a case, it is necessary to perform grayscale conversion so as to match the grayscale range of an output system, and usually, conversion is performed according to a fixed grayscale characteristic for a typical scene.
However, when an image signal having a grayscale range that is subjectively preferable for, for example, the human vision system, grayscale conversion must be performed for each area in the image signal (hereinafter, this is referred to as “space-variant grayscale conversion”.
For example, Publication of Japanese Patent No. 3465226 discloses a method of adaptively performing grayscale conversion for areas obtained by dividing an image signal on the basis of texture information.
Noise reduction is typically performed on an image signal. For example, Japanese Unexamined Patent Application, Publication No. 2004-88149 discloses a method of performing noise reduction by estimating the amount of noise in each area in the image signal.
Patent Document 1:                Publication of Japanese Patent No. 3465226        
Patent Document 2:                Japanese Unexamined Patent Application, Publication No. 2004-88149        